1. Technical Field
This disclose relates to an excess-current protection circuit to control an excess current flowing to a power supply device, and a power supply including the excess-current protection circuit.
2. Discussion of the Background
In general, excess-current protection circuits for protecting power supply systems from excessive current by limiting the amount of current flowing through the power supply system are provided.
For example, one related-art power supply circuit includes an excess-current protection circuit as shown in FIG. 11. The protection circuit 100 is formed by an excess-current control circuit 100, an input terminal 1, an output terminal 2, a driver transistor 3, a booster circuit 9, and a load 30. The excess-current control circuit 100 includes a sense resistor 5, a first sense transistor 4, a first comparator 6, an output metal-oxide-semiconductor (MOS) transistor 7, and a voltage source 8. The sense resistor 5 and the first sense transistor 4 are connected in parallel to the driver transistor 3. The voltage source 8 produces a predetermined voltage as a first bias voltage Vb1.
The first comparator 6 compares the first bias voltage Vb1 and a voltage V45 obtained by the current I4 flowing through the first sense transistor 4 multiplied by a resistance R5 of the sense resistor 5 (V45=I4×R5) and controls the output MOS transistor 7 based on the output result of this comparison. Then, the output MOS transistor 7 controls the gate voltage Vg-3 of the driver transistor 3.
FIGS. 12A and 12B are graphs of voltage-current curves of the driver transistors 3 and the first sense transistor 4. FIG. 12A shows a state before the excess current is detected, and FIG. 12B shows a state after the excess current is detected. In FIGS. 12A and 12B, a reference numeral Tr3 indicates properties of the driver transistor 3, and a reference numeral Tr4 indicates properties of the first sense transistor 4. Additionally, reference marks • indicate measured values, and reference numerals Δ show ideal values. Reference numerals M1 and M2 indicate multiplication to calculate the ideal value in accordance with a channel area ratio between the driver transistor 3 and the first sense transistor 4. The reference mark D indicates a difference between a drain-source voltage Vds-3 of the driver transistor 3 and a drain-source voltage Vds-4 of the first sense transistor 4. Reference numerals Er1 and Er2 indicate current errors due to channel-length modulation effects.
When the output current IOUT flows from the output terminal 2 to the load 30, because a voltage drop occurs at the sense resistor 5 that is directly connected to the first sense transistor 4, a drain-source voltage Vds-4 between the drain and the source of the first sense driver transistor 4 becomes smaller than the voltage Vds-3 between the drain and the source of the driver transistor 3.
More specifically, a measured current I4 flowing through the first sense transistor 4 becomes smaller than an ideal value thereof. The ideal value of the current I4 is the value of the current I3 flowing through the driver transistor 3 divided by the area ratio. For example, if a channel area ratio of the driver transistor 3 and the first sense transistor is designed to be k:1, the ideal value of the current I3 is k times the current I4.
Conversely, as shown in FIG. 12A, the measured current I3 flowing through the driver transistor 3 becomes larger than the ideal value of the current I3 calculated based on the measured current I3 and the channel area ratio
Consequently, because of the difference in the drain-source voltages Vds-3 and Vds-4, the current error from the channel-length modulation occurs, and therefore, the excess-current control circuit 100 detects the excess current at larger current than a predetermined current due to the error. In particular, with reference to Er1 compared with Er2 in FIG. 12A, when the drain-source voltage Vds-3 is in small linear region, the drain-source voltage Vds-3 has a major impact on the drain current Id and the increase in the excess current detected value is especially pronounced.
In FIG. 12B, solid lines indicate properties of the transistors 3 and 4 after the excess-current control circuit 100 detects the excess current, and dashed lines indicate the property of the transistors 3 and 4 before the detection. After the excess current is detected, the first comparator 6 outputs the output signal as an excess-current signal, thereby controlling the output MOS transistor 7, and a voltage Vgs applied to the gate of the first sense transistor 4 decreases. Therefore, gate-source voltages Vgs of the driver transistor 3 and the first sense transistor 4 decrease. As a result, the drain-source voltages Vds of the driver transistor 3 and the first sense transistor 4 increase, which transmits the current in an attempt to make the excess-current protection circuit operate normally.
That is, the respective currents value Id converges at a predetermined value calculated by the area ratio between the driver transistor 3 and the first sense transistor 4, a resistance value of the sense resistor 5, and the first bias voltage Vb1 generated from the constant-voltage source 8.
FIG. 13 is a graph of a voltage-current curve illustrating a relation between an output current TOUT and an output voltage VOUT when only the excess-current control circuit 100 is used as the excess-current protection circuit. In FIG. 13, in a power supply including the excess-current protection circuit 100 as described above, the output voltage value VOUT can be relatively small (close to 0V) and kept constant in a voltage region R5.
Additionally, in a voltage region R3, as the output voltage VOUT decreases, the current I3 assumes a value that is equal to a voltage difference (Vdif) between the input voltage VIN and the output voltage VOUT divided by a drain-source resistance Rds-3 of the driver transistor 3, and flows to the driver transistor 3. (IOUT=Vdif/Rds-3)
However, in a voltage region R4, that is, the region right before the excess current is detected, the output current IOUT jumps because the detected output current IOUT increases excessively.
In short, in this excess-current protection circuit using only the excess-current control circuit 100, the detected value of the excess current increases, which is a problem.
In view of the foregoing, there is market demand for power supplies including an excess-current protection circuit that detects an excess-current without increasing a detected value of the excess-current.